From the related art, a multitude of methods are known by now for operating a hybrid drive device, which has at least one combustion engine and at least one electric machine. In such hybrid drive devices, the combustion engine and the electric machine are normally mechanically and operatively connectible to each other by a separating clutch. In the so-called parallel hybrid, the drive shaft of the combustion engine and an input shaft of the electric machine are operatively connected to each other by the separating clutch. The vehicle control system is able to influence the separating clutch. In addition to the operating modes, hybrid driving, boost operating mode and recuperation, an appropriate actuation of the separating clutch also allows for purely electric driving, the separating clutch being open in the latter case and the combustion engine normally being switched off or stopped. Using the vehicle control system, it is possible to influence the torque or slip torque currently transmitted by the separating clutch in the slip state, for example by adjusting the contact pressure on clutch friction linings. When the slip torque occurs, the electric machine and the combustion engine run at different speeds such that there exists a slip in the separating clutch. A specified slip torque seldom corresponds to the slip torque actually transmitted by the separating clutch since varying coefficients of friction of the clutch linings produce inaccuracies in the control as a result of wear or temperature changes, hydraulic or mechanical inaccuracies in the actuation system of the separating clutch, hystereses, signal propagation times, aging and similar processes. The time at which a torque begins to be transmitted when closing the separating clutch is therefore also associated with uncertainties.
The actually transmitted slip torque, however, is important for operating the hybrid drive device. In particular when starting the combustion engine from the state of pure electric driving, which occurs by closing the separating clutch while driving, that is, while the electric machine is rotating and the combustion engine is initially at a standstill, the combustion engine is tow-started via the separating clutch. The electric machine, which acts on the drive wheels of the motor vehicle directly or via a transmission, is affected by the transmitted slip torque when closing the separating clutch. This may result in a drop in the speed of the electric machine. In addition, when the combustion engine is started for example, the combustion engine speed may be overshot as a result of the first ignition, which results in a temporary increase of the speed of the electric machine. This affects the rest of the drive train and in particular the drive wheels, which impairs the driving comfort. Knowing the transmitted slip torque, however, would make it possible, among other things, to compensate for or prevent such influences.
For detecting the actually transmitted slip torque, it is known for example to measure it with the aid of an appropriate torque sensor. This is expensive, however, and requires a corresponding installation space.